1. Field of the Invention
This invention relates to disconnectable mooring systems for loading and offloading liquid petroleum product oil tankers, floating storage (FSO) vessels, floating production storage and offloading (FPSO) systems, floating vessels for natural gas offloading, such as cryogenic liquefied natural gas (LNG) regas import terminals, and LNG transport vessels. In particular, the invention relates to an improved buoy recovery system for a disconnectable turret mooring system that permits turret-buoy connection at any compass heading.
2. Description of the Prior Art
Floating production systems are commonly used in remote offshore locations where design constraints on production infrastructure or the harsh seas make conventional fixed production platforms impracticable. Floating production systems typically include a vessel for storage and transportation of produced oil or gas from a remote offshore well to an onshore receiving terminal. To produce the oil or gas into the storage vessel, the floating production system must be moored in some manner to the offshore well. Many mooring systems are “turret” systems of one form or another, which are familiar to those skilled in the art and will only be briefly discussed herein. The mooring turret is an assembly disposed vertically through the vessel from a position above the main deck and down through the keel. Turrets are generally very large, expansive structures having a vertical height greater than that of the vessel and usually include large diameter upper and lower bearings, such that the vessel may rotate independently about the turret. Many prior art disconnectable mooring systems also require a large (approximately 10 meters or greater diameter) cone-shaped opening in the vessel bottom. Such large turret-vessel structures are expensive to construct. Furthermore, large openings in the vessel hull constructed to accommodate disconnectable mooring buoys cause significant drag and energy losses on those cargo vessels if they are required to sail a long distance from the mooring. Newer and larger high-speed LNG carrier/regas vessels tend to have a narrow flat bottom near their bow, the optimum location for buoy connection. A large hull opening is much less desirable at that position.
Numerous prior art patents disclose prior buoy mooring systems. Several of these disclose a submerged buoy that can be detachably released from a floating vessel. In these systems, the submerged buoy typically has sufficient buoyancy so that it can be raised into contact with the vessel keel with the aid of tensioning devices and wire systems or by its own bouyant force. For example, FIG. 1 illustrates in greater detail a prior art embodiment of a buoy recovery system 101 typically disposed within a disconnectable turret mooring system 100. The mooring system 100 includes two basic parts: a geostationary buoy 160 that is detachably connected to a turret assembly 120 disposed in the floating vessel 110. The buoy 160 is moored to the seabed (not illustrated) by a number of anchor legs 162 that are connected to the buoy 160 at anchor leg connectors 164, such that the buoy 160 is generally geostationary.
The turret assembly 120 is mounted in the vessel hull 110 such that the hull can rotate about the turret in response to environmental forces of wind, waves, and currents. The hull 110 opens to the sea near the keel elevation 108. The turret assembly 120 comprises a vertical turret shaft 122 which is equipped with a hollow hydraulic connector 128 at its lower end that is designed and arranged to disconnectably mate with a connector hub 166 located at the top of the buoy 160. Rubber fenders 125 are provided at the keel 108 to cushion the mooring process. A water seal 127 is provided to maintain watertight integrity of the turret compartment in the vessel 110.
The mooring arrangement 100 of FIG. 1 provides a fluid flow path between a subsea well or component thereof and the vessel when the vessel is moored to the buoy 160. The fluid transfer system (FTS) 100 includes a geostationary flexible conductor 168, a geostationary lower conductor 170, and a vessel-fixed upper conductor 174. The flexible conductor 168 spans the distance between the seabed (not illustrated) and the buoy 160 and couples with the lower conductor pipe 170. The upper conductor pipe 174, which is fixed to the vessel, is in fluid communication with the lower conductor pipe 170 and the flexible conductor 168 via fluid swivel stack 172.
When the buoy 160 is completely separated from the vessel 110, the buoy 160 is arranged and designed to sink to a neutrally buoyant position at a depth of approximately thirty-six meters below the sea level. The vessel 110 is moored to the buoy 160 by first recovering the submerged buoy 160 upwards to the hollow hydraulic connector 128 positioned at the keel 108. A mooring tensioning assembly 132 is used to pull in the buoy 160 using a pull-in line 135 attached to the mooring chain 130 of buoy 160. The pull-in line 135 is rigged from the mooring tensioning assembly 132 through a chain tensioner 136 in the turret assembly 120 to the mooring chain 130 of buoy 160. The mooring tensioning assembly 132 pulls in the pull-in line 135 until the loading sharply increases (i.e. once the buoy 160 is within a few yards of the vessel 110). The chain tensioner 136 in the turret shaft 122 is then engaged and begins slowly pulling in the geostationary buoy 160 up to the connection position. The turret assembly 120 is rotated with respect to the vessel using hydraulic drive motors (not illustrated) until the riser tubes 170 of turret assembly 120 and the riser tubes 168 of geostationary buoy 160 are properly aligned. The hollow hydraulic connector 128 is then locked in engagement with the connector hub 166, fixing the turret 120 with the geostationary 160 and mooring the vessel 110 to the seabed (not illustrated). The vessel is free to weathervane about the geostationary turret 120 in response to wind, waves, and currents.
Large turret structures are often necessary to support the weight of the turret-mounted tensioning assembly or provide an adequate spacing in which to rig the pull-in line from a vessel-mounted tensioning assembly. As shown in FIG. 1, mounting the mooring tensioning assembly 132 on the deck 107 of the turret assembly 120 requires an increase in the size of the turret structure 120 needed to support the mooring tensioning assembly 132 and requires that the product swivel stack 172 be disposed at a high elevation above the vessel deck 109.
3. Identification of Objects of the Invention
One or more embodiments of the invention are arranged and designed to accomplish one or more of the following objects:
An object of the invention is to provide a disconnectable turret mooring system with a vessel-mounted tensioning assembly which allows for connection at any compass heading, yet prevents pull-in line engagement with turret components.
Another object of the invention is to provide a disconnectable turret mooring system wherein the turret for buoy recovery is significantly reduced in size.
Another object of the invention is to provide a disconnectable turret mooring system which permits the swivel stack assembly to be positioned within the vessel between the deck and the keel.
Another object of the invention is to provide a buoy recovery system which supports the weight of the buoy during recovery but does not adversely affect the pull-in line or mooring chain during turret-buoy piping alignment operations.
Another object of the invention is to provide a buoy recovery system which minimizes the loading on the turret structure during turret-buoy piping alignment operations.
Another object of the invention is to provide a disconnectable turret mooring system which minimizes the opening in the vessel bottom.
Another object of the invention is to provide a disconnectable turret mooring system with an integrated buoy recovery system which can retrieve and release a geostationary buoy in high sea states and harsh conditions as a result of the arrangement of the buoy-to-ship interface equipment.
Other objects, features, and advantages of the invention will be apparent to one skilled in the art from the following specification and drawings.